The present invention relates to an aqueous binder dispersion for cationic electrodeposition coatings based on modified epoxy resins, and to a process for the preparation and use of the dispersion.
Cationic electrodeposition coating is a common coating process, especially for priming, in which water-dilutable synthetic resins carrying cationic groups are applied to electroconductive elements with the aid of direct current.
The use of modified epoxy resins as binders for cationic electrodeposition coatings is known (U.S. Pat. Nos. 4,104,147; 4,260,720; 395,364; 4,268,542).
The modified epoxy resins available hitherto for use in cationic electrodeposition coatings have only poor compatibility with aliphatic hydrocarbons, their elasticity is unsatisfactory, and they give films which cannot easily be overcoated and whose thickness should be further increased.
EP 0 256 020 discloses water-dilutable binders for cationic electrodeposition coatings. In order to prepare the binders, a diepoxide compound, if desired together with at least one monoepoxide compound, is converted to an epoxy resin in a polyaddition reaction carried out at from 100 to 195.degree. C. which is initiated by an initiator which reacts in a monofunctional manner and carries either an alcoholic OH group, a phenolic OH group or an SH group, and the epoxy resin is subsequently modified by means of primary and/or secondary amines or salts thereof and/or the salt of a tertiary amine, a sulfide/acid or phosphine/acid mixture and, if desired, also with a polyfunctional alcohol, a polycarboxylic acid, a polysulfide or a polyphenol.
In order to reduce the viscosity, solvents here must be added before or during addition of the amines. Accordingly, the high solvent content and low solids content are disadvantageous. In particular, the addition of solvents before/during addition of the amines means that excess solvent must be removed again after completion of the preparation of the binder dispersion.
The object of the present invention is accordingly to develop novel binder dispersions based on modified epoxy resins which do not have the abovementioned disadvantages. In particular, the dispersions should have a low solvent content. In particular, the aim is to obviate the need for distillative removal of solvents after preparation of the dispersion.